The protection of a wide range of material goods and documents from tampering, fraud, counterfeiting, theft, etc. is of rapidly growing concern. Counterfeit products, for example, can not only lead to lost revenues for the producers of the genuine goods, but can also cause significant health and safety risks. Counterfeit pharmaceuticals, for example, can be difficult to detect without the right authentication methods. Furthermore, there is a need to be able to effectively track goods as they are transported and used. Many solutions for these problems use expensive or elaborate solutions, many of which are “overt” (i.e. the presence of the security feature is clearly visible to the observer). It would be desirable to obtain a security device that can have multiple levels of security, can be simple and inexpensive to make and operate, and that can be covert (i.e., an uninitiated viewer will not necessarily know that it is present).